This invention relates to a circuit for powering fluorescent lamps and more particularly to an improved ballast circuit for simultaneously powering several lamps each at a different level of intensity.
The significantly greater efficiency of fluorescent or discharge lamps, in terms of lumens per watt, as compared to incandescent lamps has contributed greatly to the wide spread use of fluorescent lamps in office buildings, public areas and mass transit vehicles throughout the world. Ballast arrangements installed in these locations have generally provided only a single level light output while a lamp is in its operative state. This output can, in some instances, cause glare in adjacent glass or other reflective surfaces. This is especially true in a vehicle such as a bus in which one or several fluorescent lamps located in the immediate vicinity of a driver could impair the driver's visibility by causing reflections from windows or windshields which produce glare. Accordingly, it would be desirable to provide a ballast arrangement in which one or more fluorescent lamps could be powered at a lower or dimmer level than other lamps without having to use a separate ballast to dim such lamps or having to sacrifice light level output of other lamps coupled to the same ballast.
A multiple light level ballast arrangement for a fluorescent lamp is known in the art. Typically, a fluorescent lamp of conventional design is combined with a ballast circuit which can selectively introduce an added impedance into the circuit to reduce the current flow into the lamp. The reduced current flow causes the lamp to dim.
A basic method of changing the light level of a fluorescent lamp is disclosed in U.S. Pat. No. 2,350,462 to Johns. This method uses a multiply tapped secondary winding of a ballast transformer which is connected to a fluorescent lamp by means of a switch. The switch enables power to flow to the lamp from any of the several taps of the ballast winding. Because each tap of the ballast winding is coupled to the primary winding of the ballast transformer by a different number of turns, the current level provided to the lamp is directly affected by the switch setting.
In U.S. Pat. No. 4,178,535 to Miller another fluorescent light dimming circuit is disclosed, in which the light level of a fluorescent lamp is adjusted by switching from an inductor in series with the lamp to either an inductor plus a resistor in series with the lamp or a different inductor in series with the lamp.
In U.S. Pat. No. 4,358,709 to Magai the output level of a fluorescent lamp is adjusted between a low level light output and a high level light output by shunting a resistor in series with the lamp. The low level circuit includes a series resistor and an inductor in the lamp circuit.
In yet another lamp dimming circuit disclosed in U.S. Pat. No. 3,878,431 to Petrina, a pair of lamps connected in series can be dimmed in unison. This system shunts a series dimming resistor using a "triac" switching device when a high output light level is desired. The triac is normally biased to conduct current and thereby bypass the dimming resistor to provide full current to the lamps. When the triac is biased not to conduct, the dimming resistor is introduced into the lamp current circuit to reduce the lamp current.
These prior art circuits are thus merely dimming circuits to permit a single fluorescent lamp or string of lamps to operate either at high or low output level. There remains a need for a simple ballast circuit capable of driving several lamps each at a different current level to provide both high and low light output.
In accordance with the invention, several lamps may be electrically connected to a single ballast unit each at a separate one of several current output levels. This results in a separate light level output from each lamp, tile level of one lamp being independent of the others.